Hiking is a passive skill that reduces the endurance cost of moving around overland. This is very useful if you aren’t using a mount, airship, teleportation magic, or other form of transportation.

The ingame help reads:

Skill duration: random.
Type of skill : neutral skill.
Affecting stats: St.
It uses no endurance points.
One constant attribute of the wilderness that cannot be
avoided, is its vast size. Travelling through the wilderness
means travelling without the benefit of roads and even ground.
Those skilled at moving well in the wilderness can easily
outdistance those who are not, with no effort at all. Knowing
how to move swiftly takes much discipline and skill. Also, 
there are tried and true methods on how to wear your load
for minimum wear on the body.